The only empire which has ever united France and Germany (apart from a few years under Napoleon) is the one established in the 8th century by Charlemagne, the grandson of Charles Martel and son of Pepin III.

On the death of his father in 768, Charles - whose name Charlemagne is a version of the Latin Carolus Magnus (Charles the Great) - inherits the western part of the Frankish empire, a coastal strip from southwest France up through the Netherlands into northern Germany. Three years later his brother Carloman dies. Charlemagne annexes Carloman's inheritance - central France and southwest Germany. By the time of his own death, in 814, he rules much of the rest of Germany together with northern Italy.

Conversion of the Saxons: 772-804

North of the Alps Charlemagne extends his territory eastwards to include Bavaria, but his main efforts within Germany are directed against the Saxons.

The Saxons, restless Germanic tribesmen, have long plagued the settled Frankish territories by raiding from their forest sanctuaries. Charlemagne the emperor is harmed by their depredations; Charlemagne the Christian is outraged by their pagan practices. From 772 he wages ferocious war against them, beginning with the destruction of one of their great shrines and its sacred central feature - the Irminsul or 'pillar of the world', a massive wooden column believed to support the universe.

It takes Charlemagne thirty years to subdue the Saxons; not until 804 are they finally transformed into settled Christians within his empire. It has been a brutal process. Charlemagne's method is military conquest followed by forced conversion and the planting of missionary outposts, usually in the form of bishoprics. In his book of rules, the official punishment for refusing to be baptized is death.

The chronicles record that on one day some 4500 reluctant Saxons are executed for not worshipping the right god.

Holy Roman Emperor: 800

In 799, for the third time in half a century, a pope is in need of help from the Frankish king. After being physically attacked by his enemies in the streets of Rome (their stated intention is to blind him and cut out his tongue, to make him incapable of office), Leo III makes his way through the Alps to visit Charlemagne at Paderborn.

It is not known what is agreed, but Charlemagne travels to Rome in 800 to support the pope. In a ceremony in St Peter's, on Christmas Day, Leo is due to anoint Charlemagne's son as his heir. But unexpectedly (it is maintained), as Charlemagne rises from prayer, the pope places a crown on his head and acclaims him emperor.

Charlemagne expresses displeasure but accepts the honour. The displeasure is probably diplomatic, for the legal emperor is undoubtedly the one in Constantinople. Nevertheless this public alliance between the pope and the ruler of a confederation of Germanic tribes now reflects the reality of political power in the west. And it launches the concept of the new Holy Roman Empire which will play an important role throughout the Middle Ages.

The Holy Roman Empire only becomes formally established in the next century. But it is implicit in the title adopted by Charlemagne in 800: 'Charles, most serene Augustus, crowned by God, great and pacific emperor, governing the Roman empire.'

Aachen or Aix-la-Chapelle: 805

Five years after the coronation in Rome, Leo III is again with Charlemagne at a religious ceremony. But this time it is in Germany. He is consecrating Charlemagne's spectacular new church in Aachen, begun just nine years previously in 796.

The French name of Aachen, Aix-la-Chapelle, specifically features this famous building - a small but richly decorated octagonal chapel which Charlemagne has consciously modelled on another famous imperial church, Justinian's San Vitale in Ravenna.

Much is significant about the choice of Aachen as Charlemagne's seat of power. It is in the north of his empire, at the opposite extreme from Rome. The pope's journey north in 805 makes it plain that Rome cannot assume precedence in this new Christian partnership; and when Charlemagne decides to crown his only surviving son, Louis, as co-emperor in 813, the ceremony takes place in the imperial chapel at Aachen without the pope.

The site of Aachen is also ideal in terms of Charlemagne's united Frankish empire. It lies exactly between the west and east Frankish kingdoms, a fact reflected in its modern position at the intersection between the borders of Belgium, the Netherlands and Germany.

A centre of Christian learning:780-814

While extending his territories, Charlemagne needs to improve the administration of the empire. Christian clerics (the only literate group in the barbarian north) are enlisted as his civil servants at Aachen, where the emperor also establishes a programme of education and cultural revival.

Alcuin, a distinguished teacher from York, is invited in 780 to found a school in the palace at Aachen (Charlemagne and his family sometimes join the lessons); and the copying of manuscripts is carried out in a beautiful script which later becomes the basis of Roman type. Though still primitive by the standards of classical culture, the renewal of intellectual and artistic life under Charlemagne has justly been described as the Carolingian Renaissance.

The Carolingian inheritance:814

Charlemagne intends, in the tradition of the Franks, to divide his territory equally between his sons. But the two eldest die, in 810 and 811, leaving only Louis - who succeeds as sole emperor in 814. His subsequent name, Louis the Pious, reveals a character different from his father's; he is more interested in asserting authority through the medium of church and monastery than on the battlefield.

Charlemagne's great empire remains precariously intact for this one reign after his death. Its fragmentation begins when Louis dies, in 840. But the name of Charlemagne in legend and literature remains vigorously alive .

The region united by Charlemagne includes, in modern terms, northeast Spain, France, Belgium, the Netherlands, much of Germany, Switzerland, Austria and north Italy. In 840, on the death of Charlemagne's son Louis the Pious, war breaks out between his three sons over their shares of this inheritance.

A division between the brothers is finally agreed, in 843, in a treaty signed at Verdun. The dividing lines drawn on this occasion prove of lasting and dark significance in the history of Europe.

Three slices of Francia: 843

Two facts of European geography (the Atlantic coast and the Rhine) dictate a vertical division of the Frankish empire, known in Latin as Francia. The three available sections are the west, the middle and the east - Francia Occidentalis, Francia Media and Francia Orientalis.

It is clear that Francia Occidentalis will include much of modern France, and that Francia Orientalis will approximate to the German-speaking areas east of the Rhine. Francia Media, an ambiguous region between them, is the richest strip of territory. Allotted to Charlemagne's eldest son, Lothair I, it stretches from the Netherlands and Belgium down both sides of the Rhine to Switzerland and Italy.

This central Frankish kingdom is in subsequent centuries, including our own, one of the great fault lines of Europe. The northern section becomes known as Lotharingia (the territory of Lothair) and thus, in French, Lorraine; between it and Switzerland is Alsace. As power grows or decreases to the west or the east, in the great regions emerging slowly as France and Germany, these Rhineland provinces frequently change hands or allegiance.

So, for many centuries, do the Low Countries, Burgundy and northern Italy.